Field of the Disclosure
The present disclosure relates to the production of glass articles from high-softening-point silicate glasses, and more particularly to durable tools for forming such glasses at high temperatures.
Technical Background
Forming glass articles of complex shape from high-softening-point glasses is challenging due to the high forming temperatures that are necessary to successfully shape the glasses. Technical glasses of the kinds used for advanced consumer electronics applications, particularly including the refractory aluminosilicate glasses used to make cover glass sheet products for information displays (televisions, computer monitors, and mobile electronic devices such as cell phones, as well as touch screens for large and small information displays including whiteboards, pad computers, and advanced hand-held devices) can have softening points and/or forming temperatures of 800° C. or higher. At those temperatures conventional glass-forming tools such as molds and dies are rapidly damaged by surface oxidation, mechanical abrasion, chemical corrosion, and/or temperature-induced changes in mold surface composition or microstructure. These changes can become progressively more severe as the molds are repeatedly cycled from low to high temperatures during use.
Especially problematic are refractory glasses containing significant concentrations (e.g., 5% or more by weight) of alkali metal constituents such as sodium. Alkali metals are highly mobile and reactive at high glass-forming temperatures and can cause degradation of mold surfaces that unacceptably reduces the surface quality of molded glass products.
Consumer electronics manufacturers require that glass sheet products supplied for use in information displays are delivered with an optical surface finish. The economics of consumer electronics manufacture do not allow for the imposition of post-forming finishing costs, nor is such finishing even practical where sheet products with 3-dimensional curvature are required.
The cost of durable glass-forming tools is yet another factor affecting the economics of display cover glass manufacture. The need to maintain tight dimensional tolerances as well as optical surface quality in molded display products means that tools offering extended service lives are needed to enable low-cost manufacturing. Metal glass-forming tools fabricated, for example, from iron, steel, or metal alloys comprising iron, nickel, chromium or copper can provide adequate dimensional stability at high temperatures but are subject to problems such as surface oxidation at temperatures as low as 600° C., and to glass sticking through corrosive interactions with reactive glass constituents at temperatures approaching 800° C. Any of these problems can unacceptably degrade optical surface quality in the molded glass products.
Refractory non-metallic molds formed of ceramic materials such as silica, alumina, WC, TiC, TiN, SiC, SiN, or silicon nitride alloys such as Sialon can offer good high-temperature dimensional stability but are much higher in cost than metal molds. In addition, such molds can react with alkali-containing glasses at high forming temperatures, causing glass sticking that results in poor molded glass surface finish. Moreover, surface coatings such as TiAlN, TiAlN/ZrN, Al2O3 (aluminum oxide, alumina), GaN, precious metals, precious metal alloys, and precious metal-rare earth alloys that have been applied to such molds to improve glass release characteristics or corrosion resistance add even further cost, and have not generally been shown to be effective in enhancing molding performance at use temperatures above about 700° C.
Accordingly, there remains a need for economic glass-forming tooling that can be procured at low cost, that is dimensionally stable at high temperatures, and that incorporates forming surfaces capable of providing shaped products with optically finished surfaces at forming temperatures of 800° C. or higher for prolonged periods of service.